1. Field of the Invention
This invention relates to packaged electronic devices such as packaged integrated circuit chips. In particular, the invention relates to manufacture of a substrate-based packaged electronic device in which encapsulant is formed over the electronic device, such that excess encapsulant can be removed without damaging the packaged electronic device.
2. Related Art
As integrated circuits have become more complex, a need has arisen for a packaged integrated circuit having a large number of high density, reliable external package connections. It is also desirable to include in a packaged integrated circuit one or more conductive layers for signal routing and/or provision of ground and/or power planes. To meet these needs, the ball grid array has been developed.
FIG. 1 is a cross-sectional view of a typical ball grid array 100. A semiconductor die 101 is attached to a surface (die attach surface) 102a of a substrate 102, e.g., a printed circuit board (PCB), with adhesive (not shown). Electrically conductive bond pads (not shown) on the die 101 are connected with electrically conductive bond wires 103 to electrically conductive traces 102b and/or electrically conductive regions (not shown) formed on the die attach surface 102a of the substrate 102. Electrically conductive vias 102c are formed through the substrate 102 from the traces 102b and/or regions on the die attach surface 102a to a surface (mounting surface) 102d of the substrate 102 opposite the die attach surface 102a. Electrically conductive traces 102e formed on the mounting surface 102d extend to solder pads 102f formed on the mounting surface 102d. On of a plurality of solder bumps 104 are formed on each of the solder pads 102f. The solder bumps 104 are reflown to attach the substrate 102 to a larger mounting board (not shown).
An encapsulant 105 such as plastic is formed to enclose the semiconductor die 101, the bond wires 103 and a portion of the die attach surface 102a of the substrate 102 including most of the traces 102b and/or regions. The vias 102c are outside the encapsulant 105. The encapsulant 105 is frequently formed by an injection or transfer molding process.
It would be desirable to use conventional two-piece molding equipment to form the encapsulant over the die in a ball grid array. Two-piece molding equipment is already widely used for plastic encapsulation of other semiconductor devices. Thus, use of two-piece mold equipment reduces manufacturing costs since a large amount of such equipment is already in place in manufacturing facilities and the associated processes are well understood. Further, the use of two-piece molding equipment can easily be, and has been, automated, thus reducing manufacturing costs even more.
FIGS. 2A and 2B are cross-sectional views of a conventional two-piece mold 201. FIG. 2A is viewed along sectional line 2B--2B of FIG. 2B. FIG. 2B is viewed along sectional line 2A--2A of FIG. 2A.
The mold 201 includes a first mold section 202 (upper mold section as shown in FIG. 2A) and a second mold section 203 (lower mold section as shown in FIG. 2A). A recess 203a is formed in the lower mold section 203 in which a substrate 206 is positioned. A semiconductor die 207 is attached to the substrate 206. A cavity 202a is formed in the upper mold section 202 so that when the upper mold section 202 and the lower mold section 203 are brought into contact, the semiconductor die 207 is positioned within the cavity 202a.
The lower mold section 203 is formed with a pot 203b in which a pellet of solid encapsulant 205 is placed. The upper mold section 202 is formed with a hole 202d through which a transfer ram 204 movably extends. The transfer ram 204 is positioned above the encapsulant 205 in the pot 203b. A runner 202b in the upper mold section 202 extends from the hole 202d to the cavity 202a.
Though not shown completely in FIGS. 2A and 2B, another runner 202e can extend from the hole 202d opposite the location from which the runner 202b extends. The runner 202e extends to a cavity (not shown, but diametrically opposed, in one embodiment, to cavity 202a) which encloses a die (not shown) that is attached to a substrate (not shown) mounted in a recess (not shown) in the lower mold section 203, in the same manner as described above. Though two runners 203b and 203e are shown in FIG. 2A, any appropriate number of runners can extend from hole 202d to other cavities such as cavity 202a, i.e., one, three or more. Further, the mold 201 can include a plurality of pots, each pot having one or more runners extending to one or more cavities as described above.
After the substrate 206 is positioned in the recess 203a of the lower mold section 203, the encapsulant 205 is positioned in the pot 203b, and the upper mold section 202 and the lower mold section 203 are brought into contact with each other. Then the transfer ram 204 is moved (down in FIG. 2A) through the hole 202d to compress the encapsulant 205. The mold 200 and encapsulant 205 are pre-heated so that when the transfer ram 204 compresses the encapsulant 205, the liquefied encapsulant 205 is forced through the runner 202b to fill the cavity 202a. After the encapsulant 205 fills the cavity 202a, the encapsulant 205 is cured to harden, forming a packaged device. The transfer ram 204 is withdrawn, the upper mold section 202 and the lower mold section 203 are separated and the packaged device including substrate 206 is removed from the lower mold section 203.
As is apparent from inspection of FIGS. 2A and 2B, when the packaged device including substrate 206 is removed from the lower mold section 203, the encapsulant 205 not only encloses the die 207 ("package encapsulant"), but also extends along the surface of the substrate 206, where the runner 202b was located, and into the pot 203b. The excess encapsulant, i.e., encapsulant other than that necessary to enclose the die 207, must be removed. However, when the excess encapsulant is peeled away from the substrate surface, the encapsulant adheres to the substrate surface, twisting the substrate and tearing or rupturing the substrate surface, thereby causing damage to the packaged device which can be cosmetic (e.g., marring of the substrate surface) and/or functional (e.g., fracturing of the substrate; destruction of the electrically conductive traces on the substrate surface; tearing away of the solder mask on the substrate surface to undesirably expose, for instance, copper; and/or weakening or breaking of the seal between the encapsulant and the substrate surface).
Further, in production, it is desirable to integrally form a plurality of substrates in a strip having alignment holes that are located on tooling pins of a fixture, so that the packaging process (including encapsulation) can be automated. The excess encapsulant must be removed from the strip prior to further processing since, if left attached to the strip, the excess encapsulant extends past the edge of the strip prohibiting automated handling in subsequent processes. Adherence of the excess encapsulant to the substrate during removal of the excess encapsulant may cause torquing of the strip that distorts the strip and renders the strip unusable for further processing.
Thus, standard two-piece molds cannot currently be used for forming encapsulant over substrate-based packaged electronic devices as described above. Presently, the encapsulant for such devices is formed over the die using either a three-piece mold, or a modified two-piece mold such as is illustrated in U.S. Pat. No. 4,954,308 to Yabe et al.
FIG. 3 is a cross-sectional view of one type of three-piece mold 301. The mold 301 includes a first mold section 302 (upper mold section as shown in FIG. 3), a second mold section 303 (lower mold section as shown in FIG. 3), and a gate plate 308. A recess 303a is formed in the lower mold section 303 in which a substrate 306 is positioned. A semiconductor die 307 is attached to the substrate 306. The gate plate 308 is positioned in the recess 303a on the substrate 306 to surround the die 307 and define a cavity 308a that is formed when the upper mold section 302 and the lower mold section 303 are brought into contact.
Similarly to the mold 201 described above, the lower mold section 303 includes a pot 303b in which a pellet of solid encapsulant 305 is placed. Runners 303c and 308b are formed in the lower mold section 303 and the gate plate 308, respectively, to extend from the pot 303b to the cavity 308a. A transfer ram 304 is moved (down in FIG. 3) through a hole 302a in the upper mold section 302 to compress the encapsulant 305 and force the encapsulant 305 through the runners 303c and 308b to fill the cavity 308a. The encapsulant 305 hardens, the transfer ram 304 is withdrawn, the upper mold section 302 and the lower mold section 303 are separated, the gate plate 308 is removed, and the packaged device including substrate 306 is removed from the lower mold section 303.
The use of the gate plate 308 enables the excess encapsulant associated with the runners 303c and 308b to be kept off of the surface of the substrate 306, i.e., the mold 301 is top-gated. Thus, the excess encapsulant can be separated from the package encapsulant that encloses the die 307 without damaging the packaged device.
However, the use of the three-piece mold 301 is costly and presents additional problems. First, it is difficult--if not impossible--to automate a molding process using the three-piece mold 301. Consequently, the throughput, i.e., number of electronic devices encapsulated per unit time, is decreased significantly as compared to a conventional two-piece mold. Thus, use of a three-piece mold is not viable for use in high volume production. Additionally, it is not possible to form a radius at the top corner of the package encapsulant, shown as 305a in FIG. 3, with a three-piece mold. Such a radius is helpful in removing the packaged device from the mold. Finally, though molding using corner gating is desirable because corner gating provides the best mold cavity filling profile, it is not practical to use corner gating with a three-piece mold.